This invention relates to flexible foams prepared in the reaction of a polyisocyanate and a high equivalent weight compound having a plurality of active hydrogen-containing groups.
Flexible polyurethane foams have been in commercial use for many years. Their most common uses are in cushioning, as in furniture and automobiles, bedding, carpet underlayment, and other padding or cushioning applications.
Flexible polyurethane foams have been conventionally prepared by reacting components comprising a high equivalent weight polyol, a polyisocyanate and a blowing agent. Several methods of reacting these have been used. In one method, the high equivalent weight polyol is reacted with a stoichiometric excess of the polyisocyanate to form a prepolymer which is then reacted with water and optionally other polyols and blowing agents to form the foam. In another method, the polyol is mixed with the blowing agent and all the other components except the polyisocyanate to form a "B-side" composition which is then reacted with the polyisocyanate to form the foam.
In recent years it has become desirable to form higher load-bearing foam, particularly for seating materials. Several approaches to producing higher load-bearing foams have been used. One approach involves the use of cross-linkers, i.e., low equivalent weight highly reactive polyols or polyamines. Using this method, higher load-bearing foams can be prepared. Unfortunately, the use of the cross-linkers tend to upset the balance between the so-called blowing and gelling reactions which occur as the foam is produced. In order to get an open-celled foam having good physical properties, it is necessary that the reaction of the polyol with the isocyanate and the generation of gases by the blowing agent be properly sequenced. If the foam gels too quickly the foam will shrink after cooling. If the foam gels too slowly, large, uneven cells are formed, or worse, the gases produced by the blowing agent escape altogether, producing an "air bag". For this reason, the use of cross-linkers has proved difficult in commercial practice. In addition, the cross-linkers add significantly to the cost of the foam, as not only is the cost of the cross-linker added, but additional polyisocyanate must be used to compensate for the presence of the cross-linker. Accordingly, it would be desirable to use a minimal level of cross-linker in preparing flexible foams.
Another approach is to use microdispersions of polymeric filler materials. These so-called "polymer polyols" and "polymer isocyanates" provide reinforcement through the presence of colloidally sized polymer particles which are dispersed in either or both of the polyol and the polyisocyanate. A wide variety of polymer particles have been dispersed in polyols and polyisocyanates in this manner, including vinyl polymers such as styrene-acrylonitrile (SAN) copolymers, polyisocyanate polyaddition (PIPA) polymers, polyurea particles, and more recently, epoxy particles.
Several problems remain with the use of these polymer dispersions. Although they do provide improved load-bearing, as well as aiding in cell opening, it is desirable in some cases to increase the load-bearing even further. In addition, they are somewhat more difficult to handle, transport and process due to a tendency of the dispersed particles to agglomerate and settle out of the continuous phase. The polymer polyols and polymer isocyanates are expensive relative to the polyols and polyisocyanates themselves. Moreover, even with the use of these dispersions, it is usually required to use a cross-linker as well, further adding to the cost of the foam. Additionally, the problem of shrinkage of molded foams made from polymer dispersions in applications in which crushing is impossible is a further disadvantage.
A further consideration is the preparation of low density foams by the use of increasing amounts of water in the foam formulation. Foams are conventionally prepared using from about 2-4 parts of water per 100 parts by weight polyol to provide a cellular structure. Recently, lower density foams using up to 4.6 parts water have been made. Until now, these high water foams have been difficult to make and process. The water has tended to react much more quickly than the polyol, causing premature blowing and collapse of the foam.
It would be desirable to provide a flexible foam which has good load-bearing, in which the use of cross-linkers and polymer dispersions can be minimized or even eliminated, and which can, if desired, be prepared at a low density using high amounts of water in the formulation.